This invention relates to a method of producing a magnetic hard disk substrate having texturing marks formed on the surface of an aluminum or glass substrate.
A magnetic hard disk serving as a magnetic recording medium for recording data is mounted, for example, to an external memory device such as a magnetic memory of a computer. Such a magnetic hard disk has a magnetic medium formed on the surface of a substrate (herein referred to as the magnetic hard disk substrate) and data are recorded on this magnetic medium. Aluminum substrates with the surface treated with alumite or non-magnetically plated, for example, by Ni—P plating or glass plates are widely being used as a magnetic hard disk substrate.
The magnetic hard disk substrate is mirror-polished first and then subjected to a texturing process for having texturing marks formed on its surface. A magnetic layer (magnetic medium) and a protective layer are thereafter sequentially formed on the surface by a known film-making technology such as a sputtering process to produce the magnetic hard disk.
Circular indentations and protrusions (referred to as the tracks) are formed on the surface of the magnetic hard disk substrate, shaped similarly as and concentric with the texturing marks. Since these indentations and protrusions are obtained by forming the magnetic and protective layers on the indentations and protrusions originally formed on the surface of the magnetic hard disk substrate, they are higher and less sharply sloped than the indentations and protrusions on the magnetic hard disk substrate. For this reason, fine texturing marks without abnormal protrusions must be formed precisely on the surface of the magnetic hard disk substrate.
In recent years, magnetic hard disks are being made compact and their capacities are being increased. Accordingly, magnetic hard disks with higher recording densities are being required. In order to increase the recording density of a magnetic hard disk, the track pitch in the radial direction of the magnetic hard disk must be made finer and the bit density (density of recording wavelength) in the circumferential direction must be increased. In order to make the track pitch finer, however, the line density of the texturing marks on the surface of the magnetic hard disk substrate must be increased, say, to 70 lines/μm or more.
If a magnetic hard disk substrate is produced with texturing marks at such a high line density, the width of these texturing marks must necessarily be very small, and a known film-making technology such as a sputtering process must be used to cover the entire surface of such a magnetic hard disk substrate with magnetic particles. This means that it is not enough to simply make the pitch of the texturing marks very small and that it is also necessary to make the depths of the texturing marks uniform over the entire surface of the magnetic hard disk substrate. For example, texturing marks must be formed so as to make the ratio d/p of its depth d and its pitch p to be within the range of 0.01-0.3 and its average value within the range of 0.02-0.2.
In order to record and reproduce signals efficiently, furthermore, it is necessary to reduce the floating distance of the magnetic head from the surface of the magnetic hard disk, say, to less than 50 nm. In order to thus reduce the floating distance of the magnetic head, it is necessary to prevent the adsorption and collision of the magnetic head to and with the surface of the magnetic hard disk.
In order to prevent the adsorption of the magnetic head, the surface of the magnetic hard disk must be made sufficiently rough, and this is why approximately concentric circular protrusions and indentations called texturing marks are formed on the surface of the magnetic hard disk substrate by a texturing process such that not only is an appropriate degree of roughness provided to the surface of the magnetic hard disk but also a magnetic directionality is provided in the circumferential direction on the surface of the magnetic hard disk so as to improve its magnetic characteristics. In order to improve the magnetic characteristics, texturing marks must be made very fine, and this means that their pitch must be made very small and that the marks must be made at a very high line density.
In order to prevent the collision of the magnetic head and to stably keep the magnetic head at a small floating distance, it is necessary to make the textured surface of the magnetic hard disk substrate free of abnormally high protrusions, or that the average surface roughness Ra be within the range of 1-3 Å and that the maximum height of the protrusion be less than 30 Å.
Thus, the texturing process has become one of crucially important production processes for magnetic hard disks (as a magnetic recording medium) to be installed to an external memory device such as a magnetic recording device of a computer, determining the line density as well as reliability of the magnetic hard disk.
The texturing process is carried out by supplying slurry on the surface of a rotating magnetic hard disk substrate and pressing a polishing tape thereon such that approximately concentric circular texturing marks are mechanically formed on the surface of the magnetic hard disk substrate by free abrading particles. Polishing slurry having particles of one or more kinds of material such as diamond, alumina and silica dispersed in a dispersant is used. The polishing tape may be a tape of woven, unwoven, or flocked cloth or a foamed material.
In order to accurately form fine texturing marks with no abnormal protrusions on the surface of a magnetic hard disk, many studies have been made regarding the material, size and shape of the particles to be contained in the polishing slurry. As a result, diamond particles have come to be widely used since diamond has superior characteristics regarding resistance against abrasion, heat, oxidation and chemicals.
Diamond particles may be monocrystalline particles or polycrystalline particles. Monocrystalline diamond particles are polygonal particles with corners around them while polycrystalline diamond particles are spherical particles with no corners around them. Fine texturing marks can be formed on the surface of a magnetic hard disk substrate by a texturing process using these monocrystalline and polycrystalline diamond particles. Since scratches and abnormal protrusions are formed on the surface of a magnetic hard disk substrate by using monocrystalline diamond particles, polycrystalline diamond particles are usually used if diamond particles are to be used.
Conventional texturing processes by using polycrystalline diamond particles as polishing particles were carried out in two stages. As disclosed in Japanese Patent Publication Tokkai 6-150304, the first stage is carried out by using a polishing tape having relatively more abrasive polishing particles affixed thereto and the second stage is carried out by using polishing slurry having polishing particles comprising polycrystalline diamond particles with average diameter less than 1 μm dispersed therein and removing the abnormal protrusions formed on the surface of the magnetic hard disk substrate in the first stage of the texturing process by means of free polishing particles. Since polycrystalline diamond particles are nearly spherical and have not sharp corners, as explained above, their polishing power is weak, and they can be effectively used for removing such abnormal protrusions formed during the texturing process of the first stage. Texturing marks without abnormal protrusions are formed as a synergistic result of the texturing processes of the first and second stages.
This conventional technology is disadvantageous because it requires processes in two stages and hence it is both time-consuming and costly. Moreover, since the texturing process of the second stage is merely for the purpose of removing the abnormal protrusions and the line density of the texturing marks depends only on the texturing process of the first stage. Thus, it is not possible to provide fine texturing marks such as those with line density of 70 lines/μm, as being required in recent years.
Japanese Patent Publication Tokkai 11-138424 discloses another technology characterized as using polishing slurry having polycrystalline diamond particles dispersed in a dispersant in the form of primary particles. This is because the presence of flocks (coagulated, or agglomerated particles) as secondary particles in the polishing slurry was believed to cause unevenness in the size of the abrading particles such that scratches and abnormal protrusions are formed on the surface of the magnetic hard disk substrate and hence that uniform texturing marks cannot be formed. With polishing slurry using polycrystalline diamond particles in the form of primary particles for the polishing, however, texturing marks cannot be formed with a line density as high as 70 lines/μm, as is required in recent years.
It is generally known that abrading particles with smaller diameters must be used in order to form finer texturing marks and also that abrading particles with a uniform size must be used in order to form uniform texturing marks. If only the size of the abrading particles is made smaller in this prior art technology with a polishing tape made of a woven or unwoven cloth or a flocked material (planted with piles), the abrading particles easily pass through the gaps between the fibers of which the polishing tape is comprised. If a polishing tape made of a foamed material is used, on the other hand, the abrading particles can easily enter the indentations formed by air bubbles on the surface of the polishing tape. As a result, the abrading particles cannot polish the surface of the magnetic hard disk substrate uniformly and the polishing power becomes reduced either locally or all over the surface of the magnetic hard disk substrate. In other words, a surface that is uneven in roughness will result, and precisely defined uniform texturing marks cannot be formed.
Japanese Patent Publication 2002-30275 discloses still another technology related to the use of abrading particles comprising polycrystalline diamond particles and is characterized as using polishing slurry having dispersed in a dispersant agglomerated polycrystalline particles obtained by affirmatively causing the primary particles comprising polycrystalline diamond particles with diameters less than about 20 μm to be agglomerated. In other words, primary particles are agglomerated to increase the apparent unit particle diameter of the abrading particles (secondary particles), say, to 0.05-0.5 μm such that the problems of the kinds mentioned above can be obviated. Since it is the plurality of very small primary particles surrounding the abrading particles (secondary particles) that act on the surface of the magnetic hard disk substrate, furthermore, very fine texturing marks can be formed on the surface of the magnetic hard disk substrate. The secondary particles, which are relatively large, are easily broken off as the tape is pressed against the surface of the magnetic hard disk substrate and hence the scratches and abnormal protrusions on the surface of the magnetic hard disk substrate can be reduced.
In this technology, however, if polycrystalline diamond particles with diameters less than 10 nm (primary particles) are agglomerated to obtain secondary particles and if they are dispersed in a dispersant to obtain polishing slurry, the valley portions of the texturing marks become too shallow and fine texturing marks cannot be precisely formed.